This invention relates to a device for driving an EC (electrochromic) automatic dimming mirror (hereinafter simply referred to as "EC dimming mirror" or "dimming mirror") used as an interior or exterior rear view mirror of a vehicle.
An EC dimming mirror is made by disposing an EC element film on the front surface of the reflecting surface of a mirror and achieves a dimming effect against light from headlights of a succeeding vehicle while running at night by changing the amount of coloration of the EC element and thereby controlling reflectivity of the mirror. For obtaining such dimming effect, the dimming mirror is so controlled that, upon detection of quantity of incident light from the rear, the amount of coloration increases (i.e., reflectivity decreases) when the light from the rear is strong whereas the amount of coloration decreases (the color fades, i.e., reflectivity increases) when the light from the rear is weak.
On the other hand, the glare of the light to the human eye is produced by all of the surrounding light and, therefore, the control based on the light from the rear only does not match the human sense. For this reason, a control is generally made in such a manner that not only rear light but also surrounding light is detected and color imparting and color fading controls are made in accordance with quantity of the detected rear and surrounding light. More specifically, when surrounding light is strong, the glare of the light is not so strong notwithstanding that there is rear light. In this case, the sensitivity of detection for the rear light is reduced for making it harder for the EC element to be colored. When the surrounding light is weak, the glare of the light to the rear light is strong and, in this case, sensitivity of detection for the rear light is raised for making it easier for the EC element to be colored.
In the prior art EC dimming mirror, surrounding light and rear light are detected and a value of voltage applied to the EC element is obtained by arithmetic operation in accordance with a combination of the surrounding light and the rear light and the amount of coloration, i.e., reflectivity, is controlled by this voltage. This prior art method, however, requires a structure for combining detected values of the surrounding light and the rear light and obtaining a control amount of a single object of control by arithmetic operation.
For overcoming this problem, the applicant of the present application has proposed, by U.S. Pat. No. 5,486,952 (Japanese Patent Application Laid-open Publication No. Hei 7-281212), a device for driving an EC dimming mirror which enables control of the amount of coloration by controlling the surrounding light and the rear light individually as separate objects of control and thereby realizes a simplified structure of the device.
This device includes oscillation means capable of controlling duration of an "H" level and duration of an "L" level individually and separately. The duration of one level of an oscillation signal produced by the oscillation means is variably controlled in response to surrounding light quantity and the duration of the other level of the oscillation signal is variably controlled in response to rear light quantity. Polarity of drive voltage is switched in accordance with the "H" level and "L" level of the oscillation signal from the oscillation means to pulse drive the EC element and thereby control the amount of coloration of the EC element.
As shown in FIG. 2, the value of drive current for driving an EC element becomes the largest at the moment the polarity of the current is switched from one polarity to the other. For this reason, according to the above described drive method utilizing the pulse voltage, each time the level of the pulse is switched between "H" and "L" (e.g., more than 100 times per minute), current of a large value flows as shown in FIG. 3 and, as a result, the device requires a very large power consumption.
Further, according to the drive system utilizing the pulse voltage, characteristics (sensitivity curve) of a color imparting area and a color fading area as shown in FIG. 4 are provided in response to quantities of surrounding light and rear light. This sensitivity curve can be partly modified as shown by broken lines in FIG. 5 by changing value of resistance in the oscillation circuit. For totally changing the sensitivity curve (i.e., parallel shifting of the curve) as shown in FIG. 6, however, there is no alternative but to change transmittance of a filter provided on the front surface of a light sensor. Thus, total changing of the sensitivity curve by adjustment in the circuit was not possible in the prior art device.
It is, therefore, an object of the invention to provide a device for driving an EC dimming mirror which has overcome the above described problems of the prior art device and is capable of controlling the amount of coloration by controlling duration of pulse, realizing a lower power consumption, and achieving total change of the sensitivity curve by adjustment in the circuit.